The invention relates to a metal detector for detecting the presence of metal in crop material being fed into a forage harvester, and particularly, to such a metal detector having a magnetic field through which crop material passes and a pickup coils for generating a signal in response to metal passing through the magnetic field.
It is known to use a magnet and coil type detector to sense metal particles in crop material in a forage harvester to protect the cutterhead from damage and to protect livestock from ingesting pieces of metal. When the metal passes over the sensor, the metal detector stops the rotation of the feed rolls preventing the crop material containing the metal from reaching the cutterhead. For example, U.S. Pat. No. 3,972,156, issued 3 Aug. 1976 to Bennett, Jr. et al shows a forage harvester metal detector with a single conductor "figure-8" wire coil. However, such a coil may fail to detect metal which flows across the center of the magnet and coil assembly. Alternative designs are shown in U.S. Pat. No. 3,757,501, issued 11 Sep. 1973, and in U.S. Pat. No. 3,889,249, issued 10 Jun. 1975, both to Bennett, Jr. et al, wherein a forage harvester metal detector includes two separate staggered "figure-8" wire coils, each formed by a separate conductor. Such two-coil designs are expensive because they require more wire than is required by the single coil designs and because duplicate electronic circuits are required. All these prior art designs suffer from reduced sensitivity to metal which moves past the feed roll outside the outer ends of the detecting unit.
If a metal detector sensor is constructed using a single coil of wire, it will be very susceptible to stray magnetic fields such as those from overhead power lines. For this reason, the first successful metal detector sensors were constructed with two coils. The coils were wired so that voltages from a uniform field would be canceled. Using two coils connected in an opposing manner caused another problem, however. Metal crossing the sensor on a line bisecting the two coils could cause an equal voltage to be induced in each coil. The equal voltages would cancel each other leaving no signal to activate the metal detector. One metal detection system avoided this problem by using four coils and two sets of amplifiers. This solution is expensive and requires matched amplifiers to prevent the system from being more sensitive in some areas than other areas.
A metal detector system with three coils is described in EP patent application 546,509, published 16 Jun. 1993. However, in this system the three coils appear to be connected in a single phase series arrangement. Such a system is believed to be more sensitive in the middle than at its end. It would be desirable to have a metal detector which has a substantially uniform sensitivity.
In a multi-coil metal detector system it would be desirable for the operator to know over which of the coils did a triggering piece of metal pass. This would make it easier for the operator to locate the piece of metal and remove it from the feed rolls. However, none of the aforementioned multi-coil metal detection systems can provide such information.